The invention discloses a detection method of nano crystals formed on the surface of 7050 aluminum alloy. The detection method comprises the following steps: preparing multiple 7050 alloy samples, subjecting the alloy samples to extreme plastic deformation under a same technology and different technological parameters; obtaining X ray diffraction patterns through X ray diffraction; drawing a curve representing the relationship between the half-width of a same specific diffraction crystal face and the technological parameters; finding a growth inflection point in the curve, testifying the technological parameter of the extreme plastic deformation corresponding to the growth inflection point is the minimal technological parameter of nano crystals formed on the surface of 7050 aluminum alloy through a transmission electron microscope (TEM) and an electron diffraction experiment; drawing a curve representing the relationship between the half-width of a same non-specific diffraction crystal face and power density, and removing the half-width data of non-specific diffraction crystal face by comparing transmission electron microscope (TEM) and an electron diffraction experiment. A technological parameter that exceeds the minimal technological parameter is adopted to carry out extreme plastic deformation on 7050 aluminum alloy so as to form nano crystals on the surface of 7050 aluminum alloy, the detection can be performed without destroying the surface, and the method is suitable for production.